Various industrial and research applications benefit from spectrally-broadband imaging, including imaging in the deep UV (DUV) portion of the spectrum. Some applications further require immersion, cover glass correction, or operation over a wide temperature range. To be suitable for general use, microscope objectives need to be easily manufacturable, small enough to fit into a standard microscope, and have a reasonable cost.
The currently available microscope optics include those configured according to three different designs (dioptric (refractive), catoptric (reflective), and catadioptric (reflective+refractive)). Of the currently available objectives utilizing these design approaches, a very limited number are capable of good performance with high resolution below 400 nm. The optimized operation of a typical on-axis dioptric microscope objective is limited to the visible portion of the spectrum (for example, to the range from 400 nm to 700 nm or so). In case of the catadioptric design, the additional complication arises that the central obscuration present at least in a primary mirror of the catadioptric objective limits the optical transfer function of the system, resulting in lost sensitivity to certain spatial features of the sample interrogated with the microscope equipped with such objective. While the change of a design to an off-axis design may somewhat increase the width of the operational spectral bandwidth, an off-axis catadioptric objective becomes too bulky, big, and complicated to be of practical use in a standard setting.
Accordingly, there remains a need in redesign of a microscope objective and, in particular, the catadioptric objective for stable, low aberration operation across the spectral window or band ranging from DUV to the near IR optical projection system while minimizing the dimensions of the central obscuration present in the objective optics.